Noise control system



Jan. 18, 1938. R. M. SMITH NOISE CONTROLSYS'IZEM Filed June 29, 1935 IN VEN'T'OH Rogers M-S'miih -Witne-ss HT'T'ORN Y Patented Jan. 18, 1938 PATENT GFE HQE 2,105,558 NOISE CONTROL SYSTEM Rogers M. Smith, Merc to R tion of Delaware hantville, N. J., assignor adio Corporation of America, a

corpora- Application June 29, 1935, Serial No. 29,006 10 Claims. (Cl. 179171) The present invention relates to a noise control system for radio receiving apparatus and. the like, and has, for its primary object, to provide an radio receiving apparatus and the like which may be included in an audio frequency amplifier and which provides simultaneously, variable control of noise limitation and of the high frequency response of the audio frequency amplifier.

20 amplifier device therein may be varied conjointly with tone control means through operation of a single variable control device.

The invention will, however, be better understood from the following description when con- 25 sidered with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Figure 1 is a schematic circuit diagram of a radio receiving system embodying the invention;

go Fig. 2 shows a, series of curves illustrating an operating characteristic of the system in Fig. 1, and

Fig. 3 is a schematic diagram showing a modification of the circuit of Fig. 1.

5 Referring to Fig. 1, the radio receiving system,

represented therein, comprises a an audio frequency output circuit 9.

The audio frequency output circuit is coupled through a coupling condenser II and grid leak resistor l3, to an audio frequency amplifier stage'comprising a tube [5 having a cathode ll,

;- a control grid IS, a screen grid 2| and an output anode 23.

The control grid I9 receives biasing potential from a self-bias resistor 25 through the coupling resistor l3. The output anode 23 and screen grid 2| are connected through circuits 2? and 29 respectively with a variable resistor 3i having a movable contact 33.

The output anode circuit 21 of the amplifier device I5 also includes a suitable output device such as a transformer 35. The contact 33 connected with the circuits 2! and 29 is provided with a by-pass condenser 31 to the cathode.

One terminal M of the resistor 3! is connected with any suitable source of positive anode current indicated by the lead +B, the negative anode 5 supply being connected with the lead 39 as indicated at B. A by-pass condenser is provided across the supply circuit as indicated at 38. Movement of the contact 33 serves to vary the potential applied to the anode 23 and the screen grid 2|, thereby to vary the overload point of the amplifier device I5 as will hereinafter appear.

It Will be noted that the resistor terminal 43 is connected through a condenser 45 with the output circuit 21 adjacent to the anode 23 and on the anode side of the coupling device 35.

The coupling device 35 is connected with a suitable audio frequency anode circuit of the overloading point of for operating 35 near the appreciable increase in the amplifier power output. Since static signals, for example, are generally of V greater amplitude than the desired signal, the static signals are limited to practically the same power output level as the signals. Therefore, operating the amplifier tube on the knee of it s plate'yoltage-power output curve causes it to act as a cut-off or noise limitingdevice.

When the control device 3|-33 is operated to cause the contact 33 to approach the terminal 43, it will be noted that the resistance element in series with the condenser is reduced, thereby causing the condenser to load the output coupling device 35, maximum loading occurring when the contact 33 reaches the terminal 43. At this point also, the anode current is reduced to the lowest limit.

With this arrangement, since capacity loading occurs, with an inductive or resistive coupling system, high frequency response and static noises, composed chiefly of high frequency components, are further reduced by this reaction.

The static limiting system described depends, therefore, on the overloading characteristics of an electric discharge amplifier tube when. used as an audio frequency amplifier, and upon capacity loading across an audio frequency transmission circuit including inductance or other plate load used in the tube plate circuit. For coupling purposes, the overloading feature is made variable as well as the capacity loading, by means of a single resistor device 3|33 connected as a potentiometer and providing two functions simultaneously with one variable control element 33.

With the circuit arrangement shown, minimum static response with reduced output and high frequency response is provided, when the control element 33 is at the end 43 of the resistor 3| providing maximum resistance and maximum shunt capacity load across the normal anode circuit load, while full gain without noise reduction and without reduction in high frequency response is obtained when the contact 33 is moved to the terminal 4|. At intermediate points between the terminals M and 43 any degree of static reduction and output control may be obtained, the amplifier operating at the knee of a characteristic curve such as those shown in Fig. 2 for differing adjusted anode or screen potential.

Referring now to Fig. 3, the amplifier tube of Fig. 1 is shown directly connected to the loudspeaker 49 without the intermediary of an audio frequency amplifier. Also, the screen grid potential is made variable while the anode potential is fixed. The remainder of the circuit is the same as in Fig. 1, and like parts throughout and circuit leads bear the same reference numerals as in Fig. 1.

The tube |5 utilized as an output audio frequency amplifier is coupled through a suitable step-down transformer 5| with the loudspeaker device 49, the loudspeaker load being transferred to the anode circuit 21. The loading device 45 and-the variable means 3|-33 provide a variable load means for the output circuit of the amplifier tube, the load circuit being completed through the by-pass condensers 31 and 38.

It will be noted that the anode circuit is returned to the positive supply lead +B through a lead 53. The screen grid circuit 29 is connected with the variable contact 33 whereby as the contact is moved along the resistor 3| toward the terminal 43 the screen grid potential is reduced and the shunt load across the output coupling device 5| is increased.

It will thus be seen that the output amplifier tube may be controlled by varying the screen potential thereby to limit the power output, the relation between the screen grid voltage and the power output being such that the amplifier tube operates near the knee of its power output screen voltage curve, the curves in Fig. 2 indicating substantially the operating characteristics of the circuit of Fig. 3 as well as that of Fig. 1.

A variable means for simultaneously controlling the power output characteristic and the audio frequency response characteristic of an output audio frequency amplifier stage for noise suppression has the advantage that the noise response is reduced at the output end of the signal amplifying channel so that any noises developed in preceding circuits may also be removed just prior to the application of the output signals to the sound producing device.

As in the circuit of Fig. 1, the power output of the amplifier is variably controlled simultaneously with the output circuit loading which includes a frequency discriminating device such as the tone control condenser shown. In a power amplifier it is desirable that the power circuit be complete and for this reason the control of the screen potential alone is preferable, particularly where the power output is relatively high and the anode current is correspondingly increased.

I claim as my invention:

,1. The combination with an audio frequency amplifier including an electric discharge amplifier tube, of means for applying a self-biasing potential thereto, means providing an output coupling impedance therefor, means providing a frequency discriminating variable network in connection with said impedance. and means for applying an operating potential to an electrode of said device adapted tolimit the power output thereof, said means including a circuit connected through said variable network whereby said potential is simultaneously variable therewith.

2. An audio frequencyamplifier for a radio receiving apparatus and the like having a power output stage comprising an electric discharge amplifier device, an output coupling device there for, a load circuit connected therewith whereby the load is reflected into the output circuit of said device, means providing an auxiliary frequency discriminating load'for said amplifier device in connection with the output circuit thereof, and common means for simultaneously varying a potential on one of the electrodes of said device and said auxiliary load.

3. In an audio frequency amplifier, the combination of an output electric discharge amplifier device having a screen grid, a control grid, an output anode and a cathode, a sound producing device, means for coupling said sound producing device to said anode, and means for simultaneously varying the power output of said amplifier device and the frequency response characteristic of said amplifier stage comprising a variable resistance device in circuit with the screen grid, a reactance device connected in shunt relation to said output coupling device through said variable resistance device whereby the loading effect thereof on the output circuit of said amplifier device is simultaneously controllable with said screen grid potential.

4. In an audio frequency amplifienthe combination with an electric discharge amplifier de vice, of means providing a signal input circuit therefor, and means connected with the output circuit for varying the anode potential thereby to cause the device to operate on the knee of its power-output plate-voltage characteristic curve,

cuit connected with said variable means for simultaneous control therewith.

5. In a radio receiving system, an audio frequency amplifier adapted to operate in conjunction with a screen grid amplifier tube and comnection with said coupling device is simultaneously variable with the anode potential by said variable resistance device.

7. An audio frequency amplifier for radio receiving apparatus and the like including in combination a screen grid electric audio frequency amplifier stage including a is reduced in value.

10. A noise limiting system for radio receiving apparatus and the like including an output audio device through said variable control device whereby the power output of said amplifier stage and the frequency characteristic thereof are simultaneously variable to establish a predetermined output characteristic.

ROGERS M. SMITH. 

